This invention relates to the production of thiuram polysulfides from secondary amines, carbon disulfide and sulfur in the presence of an oxidizing agent.
Thiuram polysulfides are used above all as sulfur donors and accelerators in the vulcanization of rubber. These compounds are marketed, for example, under the name of thiuram tetrasulfide (tetramethyl or dipentamethylene thiuram tetrasulfide) or thiuram hexasulfide (dipentamethylene thiuram hexasulfide). The exact chemical constitution of these products is not yet known because there are no analytical techniques capable of distinguishing between mixtures of different polysulfides and mixtures of polysulfides and sulfur. In addition, thiuram polysulfides are sensitive compounds which, particularly in dissolved form, tend to eliminate sulfur.
These problems are described in detail, for example, in German Patent No. 27 25 166 with reference to the example of the material known as tetramethyl thiuram tetrasulfide. By using modern analytical techniques such as, for example, high-pressure liquid chromatography (HPLC) and gel permeation chromatography (GPC), it can be qualitatively shown that the commercial products mentioned above are not uniform compounds, but mixtures of several polysulfides and free sulfur of which the quantitative composition varies according to the production process.
In general, processes for producing thiuram polysulfides start out from the corresponding dithiocarbamates which are normally produced from a secondary amine, carbon disulfide and an alkali or alkaline-earth hydroxide.
Thus, U.S. Pat. Nos. 1,681,717 and 1,780,545 describe a process for producing thiuram polysulfides by reacting dithiocarbamates with sulfur chlorides in accordance with the following equation: ##STR1## Unfortunately, the yields obtained by this process are poor. An improved process for the production of thiuram tetrasulfides, particularly dipentamethylene thiuram tetrasulfide, on the basis of the above reaction equation using sulfur monochloride is described in U.S. Pat. No. 2,414,014. Yields of up to 95% can be obtained by this process.
However, all of these processes are attended by the disadvantage that they use corrosive, foul-smelling sulfur chlorides and that, in addition, large quantities of unusable salts are formed as problematical by-products.
A process which precludes the additional formation of sodium chloride is described in German Patent No. 27 25 166. In this process, dimethylammonium dimethyldithiocarbamate is reacted with hydrogen peroxide in the presence of carbon disulfide and sulfur to form tetramethyl thiuram tetrasulfide.
In one variant of this process which is described in German Patent No. 27 25 166, the dithiocarbamate salt to be reacted is formed in a preliminary reaction step from dimethylamine and carbon disulfide in water and the resulting aqueous solution of the dimethylammonium dimethyldithiocarbamate is subsequently further reacted with sulfur and hydrogen peroxide in the same reaction vessel to form tetramethyl thiuram tetrasulfide. Thus, according to Example 1 of German Patent No. 27 25 166, a reaction vessel is filled with water, dimethylamine and two drops of a nonionic surfactant, the solution is stirred at 25.degree. C. and carbon disulfide is added over a period of 14 minutes, during which the temperature rises to 35.degree. C. The sulfur is added in one portion, followed by the addition of water. Carbon disulfide is then added at the same time as hydrogen peroxide to the suspension obtained over a period of 60 minutes, the peroxide being added two minutes after the beginning of the addition of carbon disulfide. The end product is ultimately obtained after filtration in a yield of 90%.
Although this process is an improvement over the first-described process, it is limited in its application to the production of tetramethyl thiuram tetrasulfide. Further disadvantages include the need to use a comparatively expensive and non-selective oxidizing agent (hydrogen peroxide) and a nonionic surfactant, and the non-quantitative yield.
A less elaborate process for the production of thiuram disulfides was recently described in German Patent Application No. P 31 05 587.7, according to which secondary amines and carbon disulfide are reacted in the presence of a tertiary amine and an oxidizing agent. This process, which uses metal-containing catalysts and oxygen as the oxidizing agent, gives high yields of thiuram disulfides.
There is still a need for a simplified process for producing thiuram polysulfides in high yields from inexpensive starting compounds.